How can I check if a wheel of a vehicle collides with the floor?
1) Make the vehicle dynamic in the physics tab. And make the ground static.
In both of them activate the collision bounds under the same tab.
Afterwards press Game --> Show Physics Visualization
Now when you press P you are gonna see the collision boxes and when the vehicle drops to touch the ground the vertices that touch it are gonna become yellow.

2)Else add a collision sensor to the vehicle and select the material option and there add the material of the ground. You can just order a python script to print "Touch" everytime the sensor is positive.

3)Else add the same sensor as in (2) and also add a property. When the vehicle touches the ground have the property's value to change and make it visible during play time by activating the  (!) next to the property and the Game --> "Show Debug Properties".